What promise concerning his past does Darnay make to Dr. Manette in A Tale of Two Cities?

Darnay promises to reveal his true name to Dr. Manette. 
Charles feels guilty for hiding his true identity from Dr. Manette, since he is also a
Frenchman.  He does not feel right about marrying Lucie until Dr. Manette knows the
truth.  Dr. Manette is fully aware of his own fragile psychological condition, and
suspects that the truth of Darnay’s identity might be too much for him and cause him to
suffer a relapse.  When Darnay tries to tell him the truth, he refuses to hear it.  He
finally agrees to let Charles tell him when the two get married, because Charles
insists.  Dr. Manette knows that if he does relapse then, he will have the honeymoon to
recover without Lucie knowing what happened.